U.S. Pat. No. 4,192,155 (Gray), granted to the applicant, discloses a floating cushion connection member which has been in satisfactory use for a number of years. This connection member is used between the power head and the uppermost piece of drill pipe when connecting and disconnecting drill pipe in the drill string. Such use prevents jamming, premature wear on the threaded connections between pipe members and abrupt load transfers.
There are, however, areas of the cushion connection member which have caused problems. One area relates to the use of the washpipe extension on the piston. The washpipe extension extends axially from the piston into the bore of the outer housing shown as a pin connection. While theoretically there should be no angular movement between the two ends of the connection member, in practice with wear and continuing use, this is not so. A moment is periodically applied to the washpipe extension which results in stress fractures where it joins the piston.
A further problem relates to the bore diameter of the pin end of the connection member. In the connection member disclosed in aforementioned U.S. Pat. No. 4,192,155, the washpipe extension is received in the bore of the pin end which, therefore, dictates a bore diameter in the pin connection member which may be smaller than desired for strength and fluid flow purposes.
Yet a further problem relates to the use of the cushion material in the connection member. The cushion material in the member of aforementioned U.S. Pat. No. 4,192,155 does not entirely fill the cavity. This leads to shear problems when axial force is applied and, additionally, the cushion member does not contribute to the desired stiffness in the connection member when loading the drill bit.
Yet a further problem relates to the length of the member disclosed in aforementioned U.S. Pat. No. 4,192,155. It is desirable, because of the restricted distance between the drill head and the uppermost piece of drill pipe, to have a connection member with as short a length as possible. The washpipe extending from the piston limits the length reduction.